Is it worth getting a masters in AI from a top 100 ranked university?
This is a very vague question as well as a really vague statement. Deciding on your career based on "one person who did x said y..." shouldn't be a sole guide to anything in life. Even gathering a multitude of opinions would be useless if your sample is biased on either opinion, good or bad. Personally if I had the rep I would flag the question as it is also a duplicate...your duplicate. Anyways, I 'd like to try and answer  objectively even if I am biased towards a positive attitude by personal and peer experience.


  By worth it I mean will it push your career forward or potentially lead to a good PhD program in the future? 


There is nothing that "pushes" your career towards a set goal, other than your own ambition and effort. Receiving the MsC title won't do any good if you don't have the work to show for it. Yes a masters in A.I. is a step in the ladder of pursuing a PhD, but in order to achieve it you need to tick some boxes. There should be a specific sub-domain you target (else I believe a lifetime would not be enough to gain expertise). One should seek out the people in his network that can assist to that, be it professors, supervisors, colleagues or family. Academia does not pay that well , even directors of AI programs do not get as much as in industry thus you should decide which path is more keen to you. But most importantly you need to work hard. Often times a masters opens up mid-level/senior engineer positions, with some experience so it sure does help. Before proceeding to the second part of the question we need to define what "a good PhD" means. Does it mean getting into Stanford? Does it mean finding the right people that have the expertise at whichever university regardless of ranking? You need to decide on that. Then it is easier as there are steps for both. Not definitive ones, but in general you can carve out a plan. For example getting into Stanford would mean that you have excellent grades, a great network for recommendation letters along with published articles. Oh you need to be able to afford it too. The second would suggest that you search tirelessly for universities that have labs with high output, talk to the people in charge and decide which one is best for you.


  A post doc I spoke to about the subject said it's not worth it because the professors at not a top 100 school might not make very good mentors and you might end up wasting your time and money by learning to do things in bad way.


I am sorry to criticize this harshly, but it is among the most preposterous statements I have ever read regarding academia. Let's grab a rankings list. For example the Times one. This would mean that Karlsruhe, Eindhoven, Uppsala, Copenhagen, St Andrews (selection is arbitrary but I know that St Andrews has a great research oriented department) among others do not have a single person that would help you and they might even make you learn things the wrong way...let's be serious please.

It is true that there are bad apples, there exist everywhere. While pursuing a phd you won't attend any lectures though. You will study and do research. That means questioning everything, even your mentors. This is how science advances. Another thing you should consider is what do you expect from the PhD. Do you want to enter the industry or pursue a career in academia? Depending on your answer you should seek different institutions and different people.

I would suggest these sources too:


Ai masters Q#1
ML Learning Reddit


More importantly I would suggest taking time to talk with people from universities. Try tracking down alumni from schools you like and talk to them about their experience, people are always eager to talk about themselves. Then decide what it is that you want to do. A million stack questions want make the decision for you.

DISCLAIMER: This is a positively biased opinion based on personal experience as well as based on the notion that in order to achieve a goal one must have clearly predefined it and work hard towards it.

DISCLAIMER 2: Times just updated their rankings so the previous link is deprecated...not that it matters as most of ranking sites are marketing stunts and unis pay for them shhh